THE NERVOUS SYSTEM 157 



moderate intensity. Divide them and stimulate the distal end of each. 

 Observe that they innervate different groups of muscles. 



A similar localization of function may be detected in the sciatic 

 center itself. Very fine needle electrodes should be used in mapping 

 out this area. 



6. Summation of Afferent Impulses. Destroy the brain of a frog 

 (not the cord). Adjust two thin copper wires to one foot about 1 cm. 

 apart and connect them with the secondary coil of an indue torium. 

 Stimulate with a subminimal-reflex induction shock, i. e., with one which 

 does not evoke a reflex, but may cause a local muscular reaction. 

 Stimulate with two or three of these shocks in quick succession, and 

 observe the reflex action ultimately resulting in consequence of this 

 summation. 



7. Effect of Thermal Stimuli. Remove the wires from the frog used 

 in Experiment 6. Dip the foot at intervals into water of 10, 20, 

 and 30 C. Which is the most efficient stimulus? Finally, immerse 

 the foot in cold water, which is then heated gradually until an intense 

 reflex is evoked. 



8. Spreading of Reflexes. Apply the electrodes to the foot of this 

 frog and stimulate first with a weak current and then with a strong 

 current. Note that the weak stimulation gives rise to a perfectly local- 

 ized reflex, whereas the strong stimulation evokes, in addition, move- 

 ments of the other leg, trunk, and fofelimbs. In other words, the 

 strong stimulation causes the primary impulses to spread to other reflex 

 circuits. 



Hold the foot of one leg between your fingers. Place a small piece 

 of filter-paper moistened with moderately dilute acetic acid, upon the 

 skin of the ventral aspect of the thigh of the same leg. Observe that 

 the impulses so elicited eventually involve other reflex circuits, caus- 

 ing the opposite leg to be moved. Naturally, if the filter-paper is 

 brushed away by these movements, this result has a purely mechanical 

 cause. Immerse the frog in fresh water. 



9. Reflex Time. Suspend the frog in the usual way and stimulate 

 the sole of one foot several times with weak and strong electric cur- 

 rents. Count in each case the number of seconds elapsing between the 

 moment of stimulation and the onset of the reflex action. This interval 

 is the so-called reflex time. What is the relationship between this 

 period and the intensity of the stimulus. 



10. Inhibition of Reflexes Upon Central Paths. Expose and ligate 

 the sciatic nerve of one side. Divide the nerve distally to the ligature. 

 Apply the electrodes to the central end of this nerve. Immerse the 

 foot of the opposite leg in a weak solution of acetic acid and simulta- 

 neously stimulate the central end of the sciatic nerve with a weak 

 tetanizing current. Note the resultant inhibition of the reflex. At 

 what point of this reflex system do the impulses from the central end 

 of the opposite sciatic nerve interfere with the impulses from the foot 

 immersed in the acid? 



